I tried it for a couple hours, and it ran the batteries (three of them) down pretty well. I don't know exactly how much (our installed volt-meter gives unreliable readings), but I noticed the 3-stage charger went into boost mode immediately upon plugging in.
Trailer: We have three batteries, and three 70 or 75 watt solar panels on top. We have a large (1500 watt) inverter permanently mounted. However it was overcast and then rainy, so the solar likely wasn't doing much. Of course the truck is supplying some power but as other threads have discussed, it's not much.
Situation: We were on our way to the Outer Banks, which required going through a tunnel, so the propane had to be turned off. (We normally run with the propane off but on very hot days leave it on to keep the fridge active.) At our last stop before the tunnel, I shut off the propane and fired up the inverter and ran the fridge off it.
We planned to stop at a farmers market on the way, where I was going to switch back to propane. But, it was absolutely pouring rain when we did...so I was lazy and didn't put the fridge back on propane. The fridge ended up running on 120 volt power via inverter until we got to the campground, which was probably three or four hours. The up side of this is that since it was raining, it was cooler, and the fridge didn't have to work as hard.
Given our rig, that's probably the max I'd want to do. If you have a better charge line from the vehicle, or a sunny day when the solar is doing some good, you might be better off. On the other hand, you probably don't have three batteries like we do, and running them way down is hard on them. So, I'd suggest doing it for relatively short stints only, unless you've altered the 12 volt
system to support it.